1. Field of the Invention
This invention relates to a method of controlling the density of lines plotted by ink jet and similar plotters having printhead carriages driven by stepper motors or similar incremental means.
2. Description of the Prior Art
Ink jet printer-plotters, as shown in FIG. 1, are known in the art. Ink jet printers are readily adapted for use as plotters, in which application the printer 10 is often used for example as a real time plotter to plot data provided by a host computer 12 or instrument. Ink jet plotters operate by expelling tiny dots of ink from an ink supply through orifices 13, etc. (called nozzles) onto a medium 14 such as a piece of paper. The ink supply and orifices are typically incorporated into a print cartridge 16, which is mounted on a carriage 18. One well known ink jet cartridge is the Hewlett Packard Thermal Ink-jet print cartridge. In a typical printer, 10, the carriage 18 moves back and forth along a guide rail 20 under the control of a conventional stepper motor 22. The paper 14 is advanced through the printer by means of a conventional paper tractor typically driven by a second stepper motor 24.
The ejection of the ink droplets, the movement of the carriage, and the advancement of the paper are conventionally all under the control of a microcontroller 26 installed in the printer 10. The microcontroller 26 typically includes ROM 28 (read only memory) which stores a computer program for operation of the printer 10.
Use of such a dot type printer whose carriage and medium are moved in steps by a stepper motor is satisfactory for printing text, but poses problems when used for plotting charts, especially when the plotting is on a real time basis.
In a typical printer 1 , the carriage stepper motor 22 moves the carriage 18 back and forth along a 7.25 inch (18.4 cm) length of the guide rail 20. The carriage stepper motor uses for example 2000 steps to move the carriage this length; 2000 steps+7.25 inch equals 276 steps per inch (2.54 cm) along the guide rail. The direction of the guide rail is designated as the Y axis. However along the other axis, designated the X axis, at right angles to the guide rail, the printer prints for example 630 dots per inch (2.54 cm).
FIG. 2A illustrates the resulting deficiency of the prior art. Line segment a--a, along the X axis (630 dots per inch), is more densely printed and thus appears darker than does line b--b which is more nearly parallel to the Y axis (276 dots per inch).
Thus, this deficiency of the prior art results in plotted lines with ink densities differing from one line segment of the plot to another, depending on the angle relative to the axes of each segment. This is undesirable, especially since other kinds of plotters are available that do not use ink dots and stepper motors and so do not have these deficiencies.